1. Field of the Invention
The present invention relates to electrochemical fuel cells. In particular, the invention provides an improved seal for a membrane electrode assembly for a fuel cell, and a method of making an improved membrane electrode assembly.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) consisting of an ion exchange membrane disposed between two fluid distribution layers comprising porous, electrically conductive sheet material. The membrane is ion conductive (typically proton conductive), and also acts as a barrier for isolating the reactant streams from each other. Another function of the membrane is to act as an electrical insulator between the two fluid distribution layers. The electrodes should be electrically insulated from each other to prevent short-circuiting.
It is desirable to seal reactant fluid stream passages to prevent leaks or inter-mixing of the fuel and oxidant fluid streams. Fuel cell stacks typically employ resilient seals between stack components. Such seals isolate the manifolds and the electrochemically active area of the fuel cell MEAs by circumscribing these areas. For example, a fluid tight seal can be achieved in a conventional fuel cell stack by using elastomeric gasket seals interposed between the flow field plates and the membrane, with sealing effected by applying a compressive force to the resilient gasket. Accordingly, it is important for conventional fuel cell stacks to be equipped with seals and a suitable compression assembly for applying a compressive force to the seals.
Conventional methods of sealing around plate manifold openings and MEAs within fuel cells include framing the MEA with a resilient fluid impermeable gasket, placing preformed gaskets in channels in the electrode layers and/or separator plates, or molding seals within grooves in the electrode layer or separator plate, circumscribing the electrochemically active area and any fluid manifold openings. Examples of conventional methods are disclosed in U.S. Pat. Nos. 5,176,966 and 5,284,718. Typically, the gasket seals are cut from a sheet of gasket material. For a gasket seal that seals around the electrochemically active area of the MEA, the central portion of the sheet is cut away. This procedure results in a large amount of the gasket material being wasted. Because the fluid distribution layers are porous, for the gasket seals to operate effectively, the gasket seals ordinarily are in direct contact with the flow field plates and the ion exchange membrane. Therefore, in a conventional MEA, electrode material is cut away in the sealing regions so that the gasket will contact the ion exchange membrane. Some MEAs employ additional thin-film layers to protect the ion exchange membrane where it would otherwise be exposed in the gasket seal areas. Separate components such as gasket seals and thin-film layers require respective processing or assembly steps, which add to the complexity and expense of manufacturing fuel cell stacks.
Accordingly, it is desirable to simplify and reduce the number of individual or separate components involved in sealing in a fuel cell stack since this reduces assembly time and the cost of manufacturing.
An improved membrane electrode sealing assembly for an electrochemical fuel cell comprises:
a membrane electrode assembly having two fluid distribution layers, an ion exchange membrane and catalyst layers disposed between the ion exchange membrane and each fluid distribution layer; and
a framing seal having a groove, the groove defining two sealing portions separated by a web portion in the framing seal.
The groove of the framing seal engages the edge of the membrane electrode assembly.
The framing seal may be an elastomer such as, for example, silicones, fluorosilicones, fluoroelastomers, ethylene-co-propylene diene monomer (EPDM), natural rubber, nitrile rubber, butyl rubber, polyurethane or a thermoplastic elastomer. The framing seal may be formed by, for example, injection molding, compression molding, insert molding, etc. The framing seal may extend laterally beyond the membrane electrode assembly to form an external region having manifold openings therein. The framing seal may have manifold seals around such manifold openings as well as fluid distribution features formed therein.
A method of making such a membrane electrode sealing assembly comprises:
providing a membrane electrode assembly;
providing a framing seal having a groove, the groove defining two sealing portions separated by a web portion in the framing seal; and
fitting the membrane electrode assembly into the groove of the framing seal such that the framing seal engages the edge of the impregnated membrane electrode assembly.
A tight fit will be observed between the framing seal and the MEA if the framing seal is the same size as or slightly smaller than the MEA and thus no adhesive is necessary.
These and other aspects of the invention will be evident upon reference to the attached figures and following detailed description.